How many cubes are you able to place on 1 tower?

Before asking such questions it’s a good idea to throughly read the game manual, and search it for any terms related to your specific question. In particular, your question is answered by the definition of “placed”:

Note: Only one (1) Cube can be Placed in each Tower. If multiple Cubes meet the definition of Placed for a single Tower, then neither one is considered Placed.

I don’t think it’s even possible for 2 cubes to meet the definition of placed, so no. besides the rule prohibits it.

Rule book states if two cubes is on a tower, no cube(s) do not count.

Yea based on how it looks, it looks like only 1 cube can even fit in the tower.